1. Field of the Invention
The present invention relates to communication systems, and more specifically to a method and apparatus for estimating gain and phase imbalance in upconverting transmitters which transmit each symbol as multiple vector components.
2Related Art
Upconverting transmitters (hereafter generally referred to as “transmitters”) are often used to transmit a sequence of symbols, with each symbol containing one or more bits. Each symbol may be represented by a vector containing multiple components (“vector components”). Each of multiple carrier signals may be used to carry a corresponding component of the sequence of symbols. In general, the sequence of vector components are used to modulate the corresponding carrier signal. For example assuming a sequence of symbols are represented by vectors {a1, b1}, {a2, b2}, . . . , {az, bz}, the sequence of vector components a1, a2, . . . , az may be encoded in a first carrier signal, and the vector components b1, b2, . . . , bz may be encoded in a second carrier signal.
A receiver receives the modulated carrier signals, recovers the vector components, and generates the symbols based on the recovered vector components. A technique commonly referred to as Quadrature-Amplitude Modulation, in which two carrier signals differing in phase by 90 degrees are used, is an example of an approach which transmits symbols in the form of symbols represented by two vector components. Quadrature-Amplitude modulation is described in further detail in a book entitled, “Digital Communications”, By: Simon Haykin, Publishers: John Wiley & sons, ISBN Number: 0471629472.
In general, it is desirable that both quadrature carrier signals (encoding different respective components) have same amplitudes and have exact 90 degree phase relationship to enable a receiver to accurately recover all the vector components. However the gain and phase relationship of these carrier signals may be different in practice, for example, due to deviation of various components from ideal characteristics. In addition, amplitude imbalances in the modulating signals can also result in a similar effect as amplitude imbalance in the carrier signals themselves. In general, appropriate corrections can be applied if the imbalances in modulation are accurately estimated. At least for such a reason, it may be desirable to estimate the phase and gain imbalances in a transmitter.